For transmitting an image signal, an active cable is used. Here, the active cable means a cable having ends provided with respective connectors in each of which an active element is embedded. For example, in an active optical cable through which an optical signal is used to transmit an image signal, a light emitting element (E/O conversion section) which is an active element is embedded in a transmission-side connector, and a light receiving element (O/E conversion section) which is an active element is embedded in a reception-side connector.
Patent Literature 1 discloses an active optical cable which is used to connect a camera with a grabber (processing device) and is in conformity to the Camera Link (Registered Trademark) standard.
The active optical cable disclosed in Patent Literature 1 is a composite cable that includes an optical fiber and a metal cable and has one end provided with a transmission-side (camera-side) connector and the other end provided with a reception-side (grabber-side) connector. The optical fiber included in the composite cable is used to transmit, from the camera to the grabber, image signals X0 through X3 together with a clock signal Xclk. The metal cable included in the composite cable can be (1) a shielded line for supplying electric power from the grabber to the camera, (2) a differential line for transmitting an upstream camera control signal SerTC from the grabber to the camera, (3) a differential line for transmitting a downstream camera control signal SerTFG from the camera to the grabber, (4) differential lines for transmitting respective trigger signals CC1 through CC4 from the grabber to the camera, and (5) a differential line for connecting a camera-side micro controller unit (MCU) embedded in the transmission-side connector with a grabber-side MCU embedded in the reception-side connector.
In the active optical cable, the transmission-side connector needs to be controlled (e.g., a magnitude of an electric current to be supplied to the light emitting element embedded in the transmission-side connector is changed) by the grabber in accordance with conditions such as an internal state (e.g., a temperature in the transmission-side connector) of the transmission-side connector or an internal state (e.g., a magnitude of an electric current outputted from the light receiving element embedded in the reception-side connector) of the reception-side connector. Therefore, it is necessary to transmit internal information, which is indicative of an internal state of the transmission-side connector, from the camera-side MCU to the grabber via the grabber-side MCU or to transmit a control signal for controlling the transmission-side connector from the grabber to the camera-side MCU via the grabber-side MCU. The differential line that connects the camera-side MCU embedded in the transmission-side connector with the grabber-side MCU embedded in the reception-side connector is provided in order to achieve such communication.